pgsql: Fix oversight in FindTriggerIncompatibleWithInheritance.

Fix oversight in FindTriggerIncompatibleWithInheritance.

This function is called from ATExecAttachPartition/ATExecAddInherit,
which prevent tables with row-level triggers with transition tables from
becoming partitions or inheritance children, to check if there is such a
trigger on the given table, but failed to check if a found trigger is
row-level, causing the caller functions to needlessly prevent a table
with only a statement-level trigger with transition tables from becoming
a partition or inheritance child.  Repair.

Oversight in commit 501ed02cf.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAPmGK167mXzwzzmJ_0YZ3EZrbwiCxtM1vogH_8drqsE6PtxRYw%40mail.gmail.com
Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/afdb4cde14948cdeb83701f457ecbfc4522c1a06

Modified Files
--------------
src/backend/commands/trigger.c         |  2 ++
src/test/regress/expected/triggers.out |  8 ++++++++
src/test/regress/sql/triggers.sql      | 10 ++++++++++
3 files changed, 20 insertions(+)